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Introduction to Lock-in amplifiers
Dorothée HugFMM, 24.9.2010
Overview
Introduction to lock-in amplifiers Different buildingblocks of a lock-in Reference channel Preamplifier Singal filtering Phase sensitive detector
Conclusion
Introduction
Why using a Lock-in? Noise reduction: improve signal to noise ratio Measure rms amplitude and phase of a signal Phase sensitive detection
What is needed? An AC signal and an AC reference signal A phase sensitive detector (psd) A phase shifter
Introduction
External Internal Reference signal
phase adjustement
sample signalandpreamplifier
amplifier Filtering of sample signal
PSD
Reference channel
Preamplifier
•Single ended or differential mode•Direct or transformer mode:Choose depending on source resistance and working frequency
Direct mode:100MΩ , 30pF
Input impedance
Transformer:Very low impedance
Filtering the signal of the sample
Notch, Bandpass, Low pass and High pass filters
Notch Bandpass
Lowpass Highpass
Phase sensitive detector (PSD)
Phase Sensitive Dedector
Low pass filter
Output
Switch control
Low drift, Normal and High dynamic range
Depending on the sensitivity the system functions in a differentmode than chosen:
Sensitivity For other sensitivity system changes to
Low drift 1μV-500mV 100nv-500nV: NormalNormal 100nV-
50mV100mV-500mV: Low drift
High dyn. Range
100nV-5mV 10mV-50mV: Normal100mV-500mV: Low drift
Conclusion
A lock-in can be used to increase the signal to noise ratio and amplify small signals
It needs two ac signals of same frequency To get good signals it is important to
correctly adjust the different parameters